the head of bestbuy thinks that all who wanted a tablet pretty much got one in the past. some can't leave home or do fieldwork w/o them, and there are those who found them too redundant to the smartphone and/or the PC. for the former bunch, the tablet hard/software hasn't gone outdated, so no new tabs there; and for the disinterested latter bunch, they will dump their tablet and stay w/ phones/PCs. So tab sales are going down.
Interestingly, PC sales suffered from a very similar set of circumstances as tablets.
And, if it weren't for 2 yr phone contract plans, the relatively marginal soft/hardware improvements would have caused similar sagging of phone sales. (as contracts now are going by wayside, we may actually see the smartphone market slump).
Killing Win XP and the rise of innovative 2 in 1 touch Win 10 hardware should bring an increase in PC sales down the road. Not sure what tech innovation will boost tablers and phones (besides new children).